The FU Orionis systems are a small but remarkable class of variable young objects which undergo outbursts in optical light of 5 magnitudes or more. While the rise times for outbursts are usually very short (~ 1-10 yr), the decay timescales range from decades to centuries. The FU Ori objects also show distinctive reflection nebulae, large infrared excesses of radiation, wavelength dependent spectral types, and ``double-peaked'' absorption line profiles (Hartmann & Kenyon 1996). The frequency of these outbursts is uncertain; in recent years an increasing number of heavily extincted potential FU Ori objects have been identified on the basis of their spectroscopic characteristics at near-infrared wavelengths.

The accretion disk model for FU Ori objects proposed by Hartmann & Kenyon (1985, 1987a, 1987b) can explain the peculiarities enumerated above in a straightforward manner. Outbursts are known in other accreting disk systems and may be the result of a common mechanism (e.g., Bell & Lin 1994). The high temperature of the inner disk produces the observed F-G supergiant optical spectrum, while the cooler outer disk produces an infrared spectrum having the spectral type of a K-M supergiant. The Keplerian rotation of the disk can produce double-peaked line profiles as often observed, with peak separation decreasing with increasing wavelength of observations, since the inner hotter disk which produces the optical spectrum rotates faster than the outer cooler disk which produces the infrared spectrum.

1.1 Hot Inner disk:

In Zhu et al. 2007, we have constructed a detailed radiative transfer disk model which reproduces the main features of the spectrum of the outbursting young stellar object FU Orionis from ~4000 A, to ~ 8 micron. Using an estimated visual extinction Av~ 1.5, a steady disk model with a central star mass ~ 0.3 Msun and a mass accretion rate ~ 2x10-4 Msun/yr, we can reproduce the spectral energy distribution of FU Ori quite well. With the mid-infrared spectrum obtained by the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope, we estimate that the outer radius of the hot, rapidly accreting inner disk is ~ 1 AU using disk models truncated at this outer radius. Inclusion of radiation from a cooler irradiated outer disk might reduce the outer limit of the hot inner disk to ~ 0.5 AU. In either case, the radius is inconsistent with a pure thermal instability model for the outburst. Our radiative transfer model implies that the central disk temperature Tc> 1000 K out to~ 0.5 - 1 AU, suggesting that the magnetorotational instability can be supported out to that distance. Assuming that the ~ 100~yr decay timescale in brightness of FU Ori represents the viscous timescale of the hot inner disk, we estimate the viscosity parameter to be a~ 0.2 - 0.02 in the outburst state, con
sistent with numerical simulations of the magnetorotational instability in disks. The radial extent of the high dM region is inconsistent with the model of Bell & Lin, but may be consistent with theories incorporating both gravitational and magnetorotational instabilities.

1.2 Evolutionary stages:

The mid- to far-infrared emission of the outbursting FU Orionis
objects has been attributed either to a flared outer disk or to an infalling envelope. In Zhu et al. 2008, we revisit this issue using detailed radiative transfer calculations to model the recent, high signal-to-noise data from the IRS instrument on the Spitzer Space Telescope. In the case of FU Ori, we find that a physically-plausible flared disk irradiated by the central accretion disk matches the observations. Building on our previous work, our accretion disk model with outer disk irradiation by the inner disk reproduces the spectral energy distribution between ~4000A to ~40 micron. Our model is consistent with near-infrared interferometry but there are some inconsistencies with mid-infared interferometric results. Including the outer disk allows us to refine our estimate of the outer radius of the outbursting, high mass accretion rate disk in FU Ori as ~ 0.5 AU, which is a crucial parameter in assessing theories of the FU Orionis phenomenon. The FUor BBW 76 is also well modelled by a 0.6 AU inner disk and a flared outer disk. However, V1515 Cyg requires an envelope with an outflow cavity to adequately reproduce the IRS spectrum. In contrast with the suggestion by Green et al., we do not require a flattened envelope to match the observations; the inferred cavity shape is qualitatively consistent with typical protostellar envelopes. This variety of dusty structures suggests that the FU Orionis phase can be present at either early or late stages of protostellar evolution.

1.3 Disk Keplerian rotation:

The emission of FU Orionis objects in outburst has been identified as arising in rapidly accreting protoplanetary disks, based on a number of observational properties. A fundamental test of the accretion disk scenario is that the differentially rotating disk spectrum should produce a variation of rotational velocity with the wavelength of observation, as spectra taken at longer wavelengths probe outer, more slowly rotating disk regions. Previous observations of FU Ori have shown smaller rotation at near-infrared (~ 2.2 micron) wavelengths than observed at optical (~ 0.6 micron) wavelengths consistent with the assumption of Keplerian rotation.

In Zhu et al. 2009a, we report a spectrum from the Phoenix instrument on Gemini South which shows that differential (slower) rotation continues to be observed out to ~ 5 micron. The observed spectrum is well matched by the prediction of our accretion disk model previously constructed to match the observed spectral energy distribution and the differential rotation at wavelengths < 2.2 micron. This kinematic result allows us to confirm our previous inference of a large outer radius (~1 AU) for the rapidly accreting region of the FU Ori disk, which presents difficulties for outburst models relying purely on thermal instability. While some optical spectra have been interpreted to pose problems for the disk interpretation of FU Ori, we show that the adjustment of the maximum effective temperature of the disk model, proposed in a previous paper, greatly reduces these difficulties.